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I am trying to send email using Gmail SMTP server from java (deployed in tomcat). When doing load testing if the number of request / sec to SMTP server grows more (>20) I get this exception 

javax.mail.MessagingException: Could not connect to SMTP host: smtp.gmail.com, port: 587;

nested exception is:java.net.ConnectException: Connection timed out

Is there any limitations in access rate for Gmail SMTP server ? Should I introduce delay between each request ?

Maria
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    You could, but what are you trying to do? Send out a mailing to lots of people? If so, you should use a specialized mailing service and not Gmail, since you will run into issues (like getting on Google's blacklist). – Stefan Nov 05 '13 at 07:41
  • I am sending verification email when a user registers into my application , containing some text and verification link . In real time, the load I am generation through performance testing is very rare. I just wanted to make sure that my App is stable , but did not expect it will turn out to be an issue SMTP server. – Maria Nov 05 '13 at 08:03
  • If you want to stress test your application, I'd suggest configuring a temporary SMTP server to prevent Google from marking your IP address as a source of spam (see my answer). – Stefan Nov 05 '13 at 08:07

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After some searching, introducing a delay between every email getting sent is the way to go if you want to keep using the SMTP servers from Google. However, from what I read, a significant delay is required for these SMTP servers in order for them not to forcibly close your connection and actively drop new connections (by significant delay, I mean delays of more than 60 seconds). Load testing is out of the question this way: modern computers should be able to process sending an email within 60 seconds without any problems.

Bottom line: don't use Gmail SMTP for load testing. I would recommend setting up your own SMTP server which you can use, or maybe (temporarily) get an account at a professional mailing service. Keep in mind though, that if you host your own SMTP server, there is a possibility that your software performs the way it should, but that the SMTP server can have spam protection (by default).

Stefan
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